Self-regenerating method and system of removing oxygen and water impurities from hydrogen gas

ABSTRACT

Oxygen and water impurities are cleaned from hydrogen, which is to be stored in tanks containing a hydride-forming metallic alloy, using a novel, self-cleaning filter system. The impurity-containing hydrogen gas is first passed through a catalyst bed comprising a catalyst which is adapted to convert oxygen in the presence of hydrogen to water. The gas then passes through an adsorbent capable of adsorbing water from the gas stream, thereby substantially removing water impurities from the hydrogen gas. The purified hydrogen gas is charged into a storage tank containing a hydride-forming metallic alloy which absorbs the hydrogen by reacting therewith to form hydrogen-loaded metallic hydride. When the storage tank is to be discharged, the hydrogen-loaded metallic hydride in the tank is treated to release hydrogen gas therefrom. The released gas is passed back through the adsorbent, thereby cleaning the adsorbent of water impurities deposited therein when the tank was being charged with hydrogen. The hydrogen gas containing water impurities is then forwarded to a hydrogen utilization unit for further use of the hydrogen.

BACKGROUND OF THE INVENTION

1. Field

The invention pertains to cleaning water and oxygen impurities fromhydrogen gas which is to be stored in tanks containing a hydride-formingmetallic alloy. The water and oxygen impurities, if left in thehydrogen, would have a detrimental effect on the hydride-formingmetallic alloys in the storage tanks.

2. State of the Art

Because of the abundance of hydrogen and its relatively pollution-freeburning qualities, the desirability of developing hydrogen as an energysource has long been recognized. A major obstacle or drawback inutilizing hydrogen thus far has been the difficulty of efficiently andsafely storing hydrogen. Storing hydrogen as a liquid is costly since itrequires considerable energy to liquify the hydrogen, and transfer ofthe liquid from one container to another results in a loss to theatmosphere of much of the hydrogen. Also, containers for the liquidhydrogen must be extremely well insulated to reduce the loss of hydrogendue to vaporization or boiling. Storing hydrogen as a gas requiresextremely heavy and bulky containers and is impractical for mostpresently contemplates uses.

The use of hydride-forming metallic reactant (hereinafter defined tomean any metals, metal compounds or alloys reacting with and therebycapable of absorbing hydrogen) appears to be an attractive approach tothe storage of hydrogen. Exemplary hydride-forming metallic reactantsinclude alloys comprising at least two elements selected from the groupconsisting of iron, titanium, nickel, calcium, magnesium, manganese, andrare earth elements. Particularly advantageous alloys includeiron-titanium, lanthanum-nickel, calcium-nickel, manganese-nickel,mischmetal-nickel, and mischmetal-calcium-nickel alloys. Storage ofhydrogen in the hydride-forming reactant, i.e., forming hydrides in aprocess which is sometimes referred to as hydriding, typically involvesapplying hydrogen gas under pressure of from about 150 to 1,000 psia tothe material while dissipating the heat generated by the hydride-formingreaction. After the reactant reacts with and absorbs the hydrogen, thecontainer is sealed under pressure to maintain the reactant in the"hydrided" state until the hydrogen is needed for subsequent use.Discharging hydrogen from the storage tanks involves a processsubstantially opposite that used for storing the hydrogen, i.e.,releasing some of the pressure on the tank in which the hydride iscontained. The discharge rate of hydrogen can be increased by heatingthe hydride in the tank.

Hydride-forming reactants presently contemplated for use in storinghydrogen not only react with and absorb hydrogen but also react with andabsorb water vapor and oxygen, which are generally present withcommercial sources of hydrogen. These impurity gases form much morestable bonds with the metallic reactant than does hydrogen, and whereashydrogen can be regenerated by lowering the pressure and/or heating thereactant, oxygen and water cannot. Ultimately, the reactant will reactwith and absorb sufficient oxygen and water during successive cycles ofstoring hydrogen containing such impurities, that the reactant becomesunsuitable for storing hydrogen.

3. Objectives

One of the principal objects of the present invention was to provide anefficient system for purifying hydrogen gas of oxygen and water prior tocharging the hydrogen gas to storage tanks containing a hydride-formingmetallic, reactant. A further object of the invention was to provide asystem which was self-regenerating, i.e., that upon discharge ofhydrogen from the storage tank, the system would regenerate orreactivate itself so as to be capable of removing oxygen and waterimpurities from succeeding charges of hydrogen containing suchimpurities.

SUMMARY OF THE INVENTION

In accordance with the invention, the above objectives are achieved bycleaning the hydrogen gas of oxygen and water impurities with a novel,self-cleaning filter system. The impurity-laden hydrogen gas is passedthrough a porous bed of catalyst which is adapted to convert oxygen towater in the presence of the hydrogen. The catalyst can be selected fromthe group consisting of platinum, palladium, and nickel. The temperatureand pressure of the hydrogen gas stream during the catalyst contact arenot critical, and, advantageously, temperatures and pressures areadapted to interface with the subsequent steps in the process of thepresent system.

Following its contact with the bed of catalyst, the hydrogen gas ispassed through a porous bed of an absorbent which is capable ofadsorbing water therefrom, to substantially remove all water impuritiesfrom the hydrogen gas, including the water formed in the preceedingcatalyst contact step. The temperature of the gas as it passes throughthe bed of adsorbent is not critical; however, as most of the adsorbentsavailable for use in this step function more effectively at moderatetemperatures or lower, the temperature of the gas passing through thebed of adsorbent is preferably no greater than about 50° C. The pressureof the gas passing through the bed of absorbent is at least about 150psia. Advantageously, the pressure of the gas is commensurate with thepressure used in charging the hydrogen gas to the storage containers,i.e., typically from about 200 to 1000 psia.

The purified hydrogen gas coming from the bed of adsorbent is introducedinto a storage tank which contains a hydride-forming, metallic reactantcapable of absorbing hydrogen by reacting therewith to formhydrogen-loaded metallic hydride. Applicable hydride-forming, metallicreactants are well known in the art, and a number of exemplary reactantshave been recited hereinabove.

When all the available hydrogen has been charged to the storage tank, orwhen the capacity of the storage tank has been achieved, the flow ofhydrogen gas is discontinued. The tank is valved closed, and hydrogen inthe tank is maintained under pressure in the "hydrided" state until itis to be withdrawn for subsequent use.

When hydrogen is to be withdrawn from the tank, the metallic reactant inthe tank is treated to release the hydrogen absorbed therein. Thereleased hydrogen is withdrawn from the tank and passed back through thebed of adsorbent in reverse direction to the flow of gas therethroughwhen the storage tank is being charged with hydrogen.

As the released hydrogen gas backflows through the adsorbent, water isdesorbed from the adsorbent and released to the flow of hydrogen,thereby cleaning the adsorbent of the water which it adsorbed during theprevious step of charging hydrogen to the storage tank. For effectivecleaning of the water from the adsorbent, the pressure of the backflowof hydrogen must be less than the pressure at which the hydrogen flowedforward through the adsorbent during the charging of the storage tank.It is preferable to maintain the pressure of the backflow of hydrogen atfrom about 15 to 100 psia, depending, of course, upon the particularadsorbent being used and the pressure of the forward flow of hydrogenduring the previous charging of the storage tank. If the forward flowpressure was sufficiently high, say between about 400 to 1000 psia, thenbackflow pressures in the upper range, e.g., up to 100 psia, can be usedwhile still obtaining effective cleaning of the adsorbent. With forwardflow pressures of between about 150 to 400 psia, depending upon theparticular adsorbent used, it may be necessary to utilize backflowpressures less than 100 psia to obtain effective cleaning of theadsorbent.

The released hydrogen, following the backflow thereof through the bed ofadsorbent, contains water impurities; however such impurities have verylittle to essentially no effect on almost all subsequent processes andother uses for which the hydrogen may be employed. Thus, the hydrogengas from the bed of adsorbent can ordinarily be forwarded to a hydrogenutilization unit without concern of the water content thereof. Ofcourse, the hydrogen can be dried at the utilization unit prior to itsuse therein if such is desired; however, that is beyond the scope of andforms no part of the present invention. Further, the hydrogenutilization unit itself forms no part of the present invention, and forpurposes of describing the present invention, it is sufficient to notethat the hydrogen utilization unit could be any unit which uses orstores hydrogen, including a chemical plant which uses hydrogen as areagent or raw product, a hydrogen fueled engine, other devices designedto burn hydrogen, another storage tank, etc.

THE DRAWING

Particular embodiments of the present invention representing the bestmode presently contemplated of carrying out the invention areillustrated in the accompanying drawing, in which:

FIG. 1 is a schematic diagram showing the self-cleaning filter used inthe hydrogen fuel system of an internal combustion engine which isdesigned to operate on gaseous hydrogen fuel, and

FIG. 2 is a schematic vertical elevation of a hydrogen storage system inaccordance with the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The method and system of the present invention can be utilized insubstantially any type of application in which hydrogen is to be storedin storage tanks containing a hydride-forming metallic reactant. Asillustrated in FIG. 1, the invention is incorporated into the fuelsystem of an internal combustion engine which uses gaseous hydrogen asits fuel. In FIG. 2, the invention is shown being used in a hydrogenstorage system in which the hydrogen can be delivered to varioushydrogen utilization units, such as hydrogen burners (either catalyticor flame type), chemical units which use hydrogen as a reagent, carriergas, or raw starting material, another storage tank, or any device whichutilizes hydrogen.

In the fuel system shown in FIG. 1, an internal combustion engine 10 isadapted to operate with gaseous hydrogen as the fuel. The fuel systemfor the engine 10 comprises a storage tank 12 which is enclosed by awater jacket 14. The storage tank 12 is filled with a hydride-forming,metallic reactant which absorbs hydrogen by reacting therewith to formhydrogen-loaded metallic hydride. The metallic reactant includes any ofthe metals or alloys mentioned hereinbefore as useful for this purpose.Preferred reactants, as presently contemplated, consist of one or moreof the alloys selected from the group consisting of iron-titanium,lanthanum-nickel, calcium-nickel, and mischmetal-calcium-nickel.

A hydrogen conduit 16 connects the storage tank 12 and filter unit 18 inflow communication with each other, so that hydrogen gas can flow fromthe filter unit 18 to the storage tank 12 and vice-versa. The filterunit 18 has a water jacket 20 substantially surrounding it. The filterunit 18 is divided into two sections. The upper section is filled with aporous bed of catalyst material 22 which is capable of converting oxygenin the presence of hydrogen to water. Exemplary catalyst materialincludes platinum, palladium and nickel. The lower section of filterunit 18 is filled with a porous bed of adsorbent 24 which is capable ofadsorbing water from the hydrogen stream passing therethrough. Theadsorbent is selected from the group consisting of molecular sieves,alumina, charcoal, and silica gel. The term molecular sieves is meant toinclude synthetic or natural zeolites which are capable of separatinggaseous materials based on their molecular size and configuration. Theconduit 16 connects the lower section, i.e., the adsorbent section, offilter unit 18 to the storage tank 12, and a hydrogen supply manifold 26is connected to the upper section, i.e., the catalyst section, of filterunit 18 in flow communication therewith.

The manifold 26 has two connections. One connection has a valve 30therein and is adapted to be connected to a supply of pressurizedhydrogen for charging hydrogen to the storage tank 12. The otherconnection on the manifold 26 is connected through a valve 32 to thefuel line 34 which is in turn connected in flow communication with thehydrogen carburetor or mixer 36 on the engine 10. The carburetor 36 isadapted to mix hydrogen fuel with incoming air and feed the mixture tothe manifold of the engine 10.

A water recirculation system is provided for supplying cooled or heatedwater to the water jackets 14 and 20. Cooled water is supplied whenhydrogen is being charged to the storage tank 12, and heated water issupplied when hydrogen is being withdrawn from the storage tank 12 forutilization in the engine 10. The water recirculation system does notform part of the present invention, but is described herein because itis advantageous in combination with an internal engine used in anautomobile. The water recirculation system includes a pump 42 whichreceives water through conduit 48 from water jacket 14 around thestorage tank 12. The pump 42 pumps the water, in series, through heatexchangers 44 and 46, respectively, and to the water jacket 20 aroundfilter unit 18. The water then flows through conduit 50 from jacket 20back to jacket 14. The pump 42 is adapted to run on the electricalsystem of the engine 10 when the engine is in operation, as well as onan external supply of electricity when the storage tank 12 is beingfilled with hydrogen. Heated water from the cooling system of the engine10 is supplied to heat exchanger 44 through conduit 52 and returned tothe engine 10 through conduit 54 when the engine 10 is operating andhydrogen is being withdrawn from storage tank 12. Means are provided forsupplying cooling water to heat exchanger 46 when the storage tank 12 isbeing filled with hydrogen. The cooling water from heat exchanger 46 isdiscarded.

The operation of the fuel system shown in FIG. 1 follows two separatecycles, depending on whether hydrogen is being charged to the storagetank 12, or whether hydrogen is being withdrawn from storage tank 12 forfuel in engine 10. During the charging of hydrogen to tank 12, theengine 10 is turned off. The valve 32 in fuel line 34 is closed, and asource of pressurized hydrogen is connected to the connector 28 onmanifold 26. Cooling water is supplied to heat exchanger 46, and pump 42is connected to an external source of electricity. Valve 30 in manifold26 is opened and hydrogen flows through the catalyst bed 22 and thenthrough the bed of adsorbent 24 in the filter unit 18. From the filterunit 18, the hydrogen flows through conduit 16 to storage tank 12wherein it reacts with and is absorbed by the reactant in tank 12. Theabsorption of hydrogen in storage tank 12 results in the release ofexothermic heat which is removed from the storage tank by the coolingwater circulating through the water jacket 14 surrounding tank 12.Although the cooling of the filter unit 18 by the circulation of coolingwater through water jacket 20 is not essential, it has been found thatthe adsorbent will absorb water at maximum efficiency when it is cooled.Cooling of the catalyst section of the filter unit has essentially noeffect on the conversion of oxygen to water, and, thus, the water jacket20 could be made to surround only the adsorption section of filter unit18.

Any oxygen impurities in the hydrogen gas is converted to water as thegas flows through the porous bed of catalyst 22 in filter unit 18. Waterimpurities originally in the hydrogen gas and the water generated as thegas flows through catalyst 22 are adsorbed from the gas as the gas flowsthrough the adsorbent 24 in filter unit 18. Thus, hydrogen gas,essentially free of oxygen and water impurities, is forwarded to thestorage tank 12.

Upon the completion of charging the hydrogen gas to the storage tank 12,valve 30 in manifold 26 is closed, the external source of electricity topump 42 is disconnected, and the hydrogen supply is removed fromconnector 28. The valve 32 in the fuel line 34 is opened, and the engine10 can be operated.

When the engine 10 is in operation, hydrogen flows from the storage tank12 through conduit 16, the filter unit 18, and the fuel line 34 to thecarburetor 36. A pressure regulator is incorporated in the carburetor 36or fuel line 34 to regulate the pressure at which the hydrogen is mixedwith the air in carburetor 36. During the operation of engine 10, heatedwater from the engine cooling system is circulated through heatexchanger 44, and pump 42 operates from the engine's electrical system.Heated water then flows through the water jackets 20 and 14 whichsurround the filter unit 18 and storage tank 12, respectively. Theheated water supplies sufficient heat to the reactant in the storagetank 12 to supply endothermic heat required in releasing hydrogen fromthe reactant. The hydrogen released from the reactant in the storagetank 12 flows back through the filter unit 18 at a substantially reducedpressure in comparison to the pressure used in charging the tank 12.Under the conditions of reduced pressure and increased temperature, theadsorbent 24 in the filter unit 18 releases water contained therein tothe flow of hydrogen gas. Thus, the adsorbent 24 is cleaned andconditioned for subsequent use in removing water from hydrogen which isbeing charged to the storage tank 12.

Following backflow through the adsorbent 24, the released hydrogen flowsthrough the catalyst section of filter unit 18 and then through fuelline 34 to the carburetor 36 of engine 10. The gas is mixed with air inthe carburetor 36, and the mixture is subsequently burned in thecylinders of the engine 10. The water content of the hydrogen has noadverse effect on the operation of engine 10. In fact, injection ofwater with the hydrogen fuel has been shown to be beneficial (see U.S.Pat. No. 3,983,882).

The self-cleaning filter unit 18 provides hydrogen essentially free ofoxygen and water to the storage tank 12. By positively eliminating theoxygen and water contaminants which are present in substantially allcommercially prepared hydrogen, the operation of the metallic reactantis troublefree and can be continued over long periods of time.Conversely, if the oxygen and water were not removed from the hydrogen,the operation of the metallic reactant would be impaired, and thereactant would function effectively for much shorter periods of time.

A more generalized use of the present invention is shown in the hydrogenstorage system illustrated diagramatically in FIG. 2 of the drawing.This storage system comprises a composite filter means consisting of anelongate, enclosed chamber having top, bottom, and side walls. As shown,the chamber is formed by the cylindrical container 56. The lower sectionof the chamber or container 56 is filled with a porous bed of adsorbent58. The adsorbent 56 is the same material as the adsorbent 24 containedin the filter unit 18 of FIG. 1. The upper portion of container 56 isfilled with a porous bed of catalyst 60. The catalyst 60 is the samematerial as the catalyst 22 contained in filter unit 18 of FIG. 1.

A dip-tube 62 extends through the top wall of the container 56, throughthe catalyst section, and at least through the major portion of theadsorbent section, with the open, lower end of the dip-tube beingpositioned within the container 56 and with the upper end of thedip-tube 62 being open to the outside of the container 56. The term"open lower end" is meant to encompass a series of openings in theportion of the dip-tube 62 positioned in the lower half of adsorbent 58as well as or in addition to a single opening in the very end of thedip-tube 62.

The upper end of the dip-tube 62 is connected in flow communication withtank 64 by conduit 66. Conduit 66 includes two flow regulators 68connected in parallel. One of the flow regulators allows fluid flowtoward the tank 64 at pressures of from about 150 psia to about 1000psia. The other flow regulator allows fluid flow from the tank 64 to thecontainer 56 at pressures of from about 15 psia to 100 psia. The tank 64is filled with a hydride-forming, metallic reactant 70. The reactant 70is the same material as the reactant contained in the tank 12 of FIG. 1.A heat exchange tube 72 is disposed in tank 64, and means are providedfor passing either cooled or heated water through tube 72. A layer ofinsulating material 74 can cover the tank 64 as shown.

Manifold means 76 is connected to the top of container 57 in flowcommunication with the bed of catalyst 60 therein. The manifold 76 hastwo connections therein. One of the connections is adapted to beconnected to a supply of hydrogen having a pressure of from about 150psia and 1000 psia. A valve 78 is provided in this connection to controlthe flow of hydrogen therein. The other connection is adapted to providehydrogen at a pressure of from about 15 psia to 100 psia to a pipelinewhich delivers the hydrogen to a hydrogen utilization unit which is notshown in the drawing. A valve 80 is provided in this connection also.

The operation of the system shown in FIG. 2 follows two separate cycles,depending upon whether hydrogen is being charged or withdrawn fromstorage tank 64. When hydrogen is to be charged to the storage tank 64,the valve 80 is closed, a source of hydrogen is connected to theappropriate connector on the manifold 76, and the valve 78 in thatconnector is opened. Hydrogen flows through the manifold 76 and into thetop of container 56. The hydrogen flows through the bed of catalyst 60wherein any oxygen impurities in the hydrogen are converted to water.From the catalyst, the hydrogen flows through the bed of adsorbent 58,wherein water impurities contained in the hydrogen are adsorbed by theadsorbent. The purified hydrogen flows through dip-tube 62, flowregulator 68, and conduit 66 to storage tank 64, wherein it reacts withand is absorbed by the reactant 70. Exothermic heat of reaction andabsorption is removed from the reactant 70 in storage tank 64 bycirculating cooling water through the heat exchange tube 72. When thedesired amount of hydrogen has been charged to the storage tank 64, thevalve 78 is closed, thus sealing the hydrogen in tank 64.

During discharge of hydrogen from storage tank 64, heated water iscirculated in the heat exchange tube 72, and valve 80 is opened allowinghydrogen to flow to wherever it is to be used. The pressure on thereactant 70 is lowered and hydrogen is released from the reactant.Transfering heat to the reactant 70 hastens the release of hydrogentherefrom. The released hydrogen flows through conduit 66, flowregulator 68, and dip-tube 62 into the bed of adsorbent 58 in container58. The backflow of hydrogen through the bed of adsorbent at a reducedpressure in comparison to the pressure employed during the charging oftank 64 results in the release of the adsorbed water to the backflow ofhydrogen thus cleaning the bed of adsorbent 58 of the water which itcollected during the charging of the tank 64. The released hydrogen thenflows through the bed of catalyst, the manifold 76, and valve 80 into apipeline which directs the hydrogen to its ultimate point of use.

A water jacket could be provided around tank 56 similar to the jacket 20around unit 18 of FIG. 1, if desired, and the flow of water coming fromtube 72 in tank 64 could then be circulated through that jacket tooptimize the adsorption and release functions of the adsorbent 58.However, it has been found that in many applications, depending uponsize of the equipment and the ambient conditions, external cooling andheating of the adsorbent 58 are unnecessary.

Whereas, this invention is described with respect to particularembodiments, it is to be understood that changes may be made therein andother embodiments constructed without departing from the novel inventiveconcepts set forth herein and in the claims which follow.

I claim:
 1. In the storage of hydrogen in tanks containing ahydride-forming metallic reactant, a self-regenerating method ofremoving oxygen and water impurities, which would otherwise have adetrimental effect on the hydride-forming metallic reactant in thestorage tanks, from the hydrogen gas being charged to the storage tanks,said method comprising:(a) passing the impurity-containing hydrogen gasthrough a porous bed of a catalyst which is adapted to convert oxygen towater in the presence of hydrogen, (b) passing the gas from step (a)through a porous bed of an adsorbent at a pressure of at least about 150psia, said adsorbent being capable of adsorbing water from the hydrogengas stream, thereby substantially removing water impurities from thehydrogen gas, (c) feeding the hydrogen gas from step (b) to a storagetank containing a hydride-forming metallic reactant which absorbs thehydrogen by reacting therewith to form hydrogen-loaded metallic hydride,(d) discontinuing the flow of hydrogen gas in steps (a), (b) and (c),(e) treating the hydrogen-loaded metallic hydride in the storage tank torelease hydrogen gas therefrom, (f) passing the released hydrogen gas ata pressure of less than about 100 psia back through the porous bed ofadsorbent used in step (b), thereby cleaning the bed of adsorbent ofimpurities deposited therein during step (b), and (g) forwarding thereleased hydrogen gas to a hydrogen utilization unit.
 2. A method inaccordance with claim 1, wherein the impurity-containing hydrogen gas ispassed through the bed of adsorbent in step (b) at a pressure of fromabout 150 to 1000 psia and the hydrogen gas is passed back through thebed of adsorbent in step (f) at a pressure from about 15 to 100 psia. 3.A method in accordance with claim 2 wherein the treatment of themetallic hydride in step (e) comprises reducing the pressure on thehydride and heating the hydride.
 4. A method in accordance with claim 1,wherein the catalyst utilized in step (a) is selected from the groupconsisting of platinum, palladium, and nickel, and the absorbentutilized in step (b) is selected from the group consisting of molecularsieves, alumina, charcoal, and silica gel.
 5. A method in accordancewith claim 1, wherein the hydride-forming metallic reactant comprises atleast two elements selected from the group consisting of iron, titanium,nickel, calcium, magnesium, manganese, and rare earth elements.
 6. Amethod in accordance with claim 5, wherein the hydride-forming metallicreactant is selected from the group consisting of iron-titanium alloys,lanthanum-nickel alloys, calcium-nickel alloys,mischmetal-calcium-nickel alloys.
 7. A method in accordance with claim1, wherein the treatment of the metallic hydride in step (e) comprisesreducing the pressure on the hydride and heating the hydride.
 8. Amethod in accordance with claim 1, wherein the temperature of thehydrogen gas stream in step (b) is no greater than about 50° C.
 9. Amethod in accordance with claim 1, wherein the released hydrogen gas instep (f) is passed first through the bed of adsorbent and then throughthe catalyst bed, and the gas coming from the catalyst bed is thenforwarded to the hydrogen utilization unit in accordance with step (g).